1. Field of the Invention
The present invention relates to an electrode substrate for a battery, and an electrode for a battery. Particularly, the present invention relates to an electrode substrate and an electrode for a battery, having high capacity maintaining rate and superior in high rate discharge property.
2. Description of Related Art
Conventionally, alkaline secondary batteries as well as lead storage batteries and lithium ion batteries achieve widespread use as secondary batteries for portable applications, mobile usage, industrial usage, and the like.
The alkaline secondary battery is used in various fields by virtue of its high reliability, long lifetime, and its advantage over the lithium ion battery in terms of reducing the cost, size, and weight. Particularly, from the standpoint of saving energy and promoting environmental conservation, automobile manufacturers are now adapting alkaline secondary batteries to practical use for hybrid vehicles and the like, attracting widespread attention even abroad. At present, a nickel-hydrogen battery is the mainstream of the power source.
For alkaline batteries employed as the power source of various devices in applications ranging from portable equipment to large facilities for industrial use, a nickel electrode is often employed as the positive electrode, i.e. the cathode. Similar to electrodes of other batteries, the nickel electrode employs a structure in which the current collector functioning to collect current carries the active cathode material for the positive electrode to induce battery reaction. In this connection, the invention of a sintered nickel plate having nickel powder sintered instead of the conventional pocket type played a major role in the development of alkaline secondary batteries for practical use.
From then on, there have been intensive efforts to reduce the cost and increase the capacity of the nickel electrode. In connection with reducing the cost, there has been proposed a two dimensional structure such as of punched metal instead of the sintered compact having a three dimensional mesh structure. Specifically, this technique is directed to producing a nickel electrode by filling the pores of the punched metal with active material paste (paste mixture including active material). However, such a nickel electrode has not yet arrived at the stage of practical use due to various problems thereof.
Increasing the capacity of the nickel electrode has become possible by employing nickel foam of a three dimensional mesh structure instead of a sintered compact. Nickel foam is generally produced by a method including the steps of applying nickel plating on a foam sheet of urethane resin, burning urethane resin, and then effecting annealing under reducing atmosphere to improve the strength of the nickel frame. Further, the pores of the nickel foam are filled with active material paste, and then subjected to pressurization. Thus, a nickel electrode is obtained. The porosity of the nickel foam (the ratio of the pores to the volume of the whole) is 92% to 96%, which is extremely high as compared to approximately 80% for a sintered compact. Since the amount of active material filled per unit volume is increased, higher capacity can be realized.
At the initial stage of development, nickel foam had the problem that it was readily damaged. For example, when a nickel electrode in sheet form was rolled up and stored in a cylindrical battery container, cracks appeared in the nickel foam. Such a problem, however, has been overcome, and a cylindrical or rectangular solid nickel-hydrogen battery employing a nickel foam current collector has been put into practical usage for portable equipment as well as for hybrid vehicles demanding high power and high reliability. Although some devices employ a nickel-hydrogen battery with a sintered compact as the nickel electrode, the mainstream has changed to an electrode having the nickel foam current collector filled with active material paste (active material mixture), such as the nickel electrode plate disclosed in Japanese Patent Laying-Open No. 09-306484, for example.
Nickel foam has now arrived at a level suitable in property from the standpoint of high power, not to say high capacity, for a current collector of an electrode destined for batteries. The remaining issue is to render the nickel electrode inexpensive by reducing the amount of nickel that corresponds to most of the cost of the nickel electrode.
At the early stage of development, the mass per unit area of nickel for nickel foam applied to a battery electrode was 500 g/m2 to 600 g/m2. There has now been developed nickel foam applicable to practical usage even if the mass per unit area is approximately 350 g/m2. However, the strength of the nickel electrode will be degraded if the amount of nickel is further reduced. Even if nickel foam can be produced, the possibility of fracture during the fabrication step of the nickel electrode or during production of the battery is extremely high.
There has been proposed a nickel electrode employing a porous electrode substrate formed by plating the surface of a nonwoven fabric core with nickel as the current collector instead of nickel foam. This nickel electrode with a nonwoven fabric as the core can have the amount of nickel reduced while maintaining a predetermined strength, as compared to the nickel foam disclosed in the aforementioned Japanese Patent Laying-Open No. 09-306484, and is advantageous in that fabrication is facilitated. An electrode formed having such an electrode substrate filled with an active material is disclosed in, for example, Japanese Patent Laying-Open No. 2005-347177.
This publication discloses a nickel electrode destined for an alkaline battery, obtained by having the surface of a nonwoven fabric made of resin plated with nickel to form a current collector (porous electrode substrate), filling the current collector with active material mixture, followed by pressure forming.
However, the battery such as of the type disclosed in the aforementioned Japanese Patent Laying-Open No. 2005-347177 is disadvantageous in that the battery performance is readily degraded in accordance with the repetitive charging and discharging.
This arises from the fact that, in the case where the level of the pressure, when applied to the electrode substrate filled with active material mixture is high, the fiber cross point of the nonwoven fabric qualified as the core is displaced after compression. If the fiber cross point is modified between the state prior to compression and after compression as in this case, the three dimensional mesh structure of the electrode substrate will fracture to cause a crack in the nickel coated on the surface of the fiber and/or constriction of the pore in the electrode substrate, leading to degradation of the battery performance.
Although the aforementioned Japanese Patent Laying-Open No. 2005-347177 teaches that the fibers of the nonwoven fabric qualified as the core is fusion-bonded at the cross point by heat treatment, it is silent about a specific definition of the degree of fusion-bonding. There is a possibility of the bonding of the fibers being broken in the compression stage of the electrode substrate to result in damage of the three dimensional mesh structure of the electrode substrate. Furthermore, it is known that the electrode bulges in accordance with the charging and discharging of the battery. There is a possibility of the three dimensional mesh structure of the electrode substrate being damaged by this bulging of the electrode.